Safety and durability are of primary concern in the production of lawn mower blades, agricultural and off-highway rotary cutter blades. Experience with blade failures in the field has driven manufacturers to produce blades with low hardness to prevent catastrophic impact failure and related safety concerns. A reduction in blade hardness will tend to reduce the incidence of brittle failures, but it will also reduce yield strength and increase failures from bending, fatigue and wear.
Rotary cutter blade standards have been developed to provide desired levels of blade performance and safety. An industry standard of relevance to rotary cutter blades is the "Blade Impact Test" of ASAE S474, Agricultural Rotary Mower Safety. This test drops a mower with blades rotating onto a two-inch diameter steel stake. No part of the mower or mower component can fail in a way hazardous to anyone in the area. This test will determine the weak link, if any, in the machine and impacts the blade in a worst case manner.
Despite industry standards, rotary cutter blades may still fail, even when those blades satisfy standards with respect to material grade, heat treat process, and hardness and bend tests for ductility. Failures due to wear and bending beyond a usable shape are the most common. These failures are often the result of specifications which tend toward low blade hardness and high ductility at the sacrifice of yield strength and wear resistance. These type of failures cause inconvenience and expense. Fatigue failures can be reduced by blade design and by increased hardness. Increased hardness will also reduce failures due to bending and wear. Mitigating the benefits of increased hardness in conventional blades, is the tendency of the higher hardness material to suffer unacceptable catastrophic impact failure.
High carbon steels exhibit desirable higher levels of hardness, but present several drawbacks in rotary blade applications, such as in a mower or cutter. High carbon steels are difficult to work, and cause accelerated tool wear, adding to manufacturing costs. Furthermore, the higher hardness of the high carbon steels is coupled with reduced toughness. In addition, higher carbon and alloy content steel is more costly, and the annealing required adds further costs to the finished blade.
Boron steels, such as 10B38, have been used for lawn mower blades in lawn mowers. Boron steels exhibit desirable high levels of toughness, but in prior art mower blades, have been susceptible to wear at a greater than optimum rate. These blades also have less than optimal resistance to edge deformation, bending, and fatigue, and do not perform as well in these respects as traditional higher carbon and alloyed steels.
Although the cost for a mower blade is small compared to the equipment cost, replacement of blades is a time-consuming operation. Hence the time between blade replacements is best extended as much as possible.
What is needed is a rotary cutting blade which presents high hardness to increase wear life, while at the same time exhibiting acceptable toughness levels to insure satisfactory operation and passage of standard blade impact tests.